This invention relates to a testing apparatus useful in calibrating a wristwatch, such as a digital watch, which includes a crystal-controlled oscillator serving as its timing standard.
One of the factors that has led to the great demand for quartz watches is their ability to operate with high accuracy. It is now reasonable for a consumer to expect his quartz watch to neither gain nor lose more than about one-half second per day. Otherwise expressed, their accuracy is about 6 parts per million (ppm) or better.
The timing accuracy of the watch is directly dependent on the frequency of a crystal-controlled oscillator in the watch. The output of the watch oscillator serves as a clocking signal for an integrated circuit multi-stage binary counter that serves as a frequency divider. In a digital watch, display elements such as LED's are illuminated in accordance with the count defined by the counter. It has become fairly standard practice in the industry for the watch oscillator to be designed for operation at 32,768 Hz. (At least one manufacturer provides an oscillator that is designed to operate 24 times higher, i.e., 786,432 Hz.)
It is also a fairly standard practice in the industry to provide an adjustable component such as a trimming capacitor as part of the watch oscillator. By adjusting the trimming capacitor, the oscillator operating frequency can be adjusted so that it is extremely close to the desired or design center frequency. A practical limit is set on this in that temperature variations, aging, and like factors affect the stability of the operating frequency. Because of these factors, it is more than acceptable if this watch oscillator is adjusted against a calibration standard whose own accuracy is approximately 0.5 ppm.
In order to know how much and in which direction such adjustment should be made, it is necessary to ascertain how close the operating frequency of the oscillator is to the desired frequency, whether it be the 32,768 Hz design center or the 786,432 Hz design center. This problem is a particulrly acute one for a small business where digital watches are sold and repaired. The jeweler is interested in being able to adjust the trimming component within the watch so that the timing standard meets desired specifications as to design center frequency. This could be done in many different ways, particularly if the jeweler had a large budget for purchasing expensive electronic measuring equipment. It is another matter, of course, to provide a relatively inexpensive, easy to operate, adequately accurate instrument to facilitate the adjustment.
A variety of approaches have been proposed with respect to this problem. One aspect of the problem which has acceptably been overcome in prior art approaches relates to sensing the frequency of the watch oscillator without loading it. In U.S. Pat. No. 3,892,124, for example, there is described an analyzer which employs a technique whereby electromagnetic radiation from the watch oscillator is picked up by an antenna and then amplified. In this prior art analyzer, the amplified signal and a reference signal are supplied to a mixer. The reference signal is generated by a crystal-controlled reference oscillator in the analyzer. A difference or beat frequency is produced by the mixer to provide a measure of the difference in frequency between the frequency of the watch oscillator and the frequency of the reference oscillator. The foregoing arrangement suffers from the disadvantage that it is relatively complex, and is accordingly costly to manufacture.
Other prior art instruments have other shortcomings. In some calibration instruments, a digital counter is used to effect a frequency measurement by counting during predetermined measurement intervals. The measurement intervals are defined repetitiously whereby the overall measurement process can be considered to be a sampling process. The problem here is that the calibration instrument has a slow response time. This makes it difficult for the jeweler to adjust the watch oscillator frequency to the desired value. In particular, there is a tendency to overshoot, in one direction or another, the desired frequency during the time that the sampling period is being defined.